The present invention relates to a defect inspection apparatus for a phase shift mask. More particularly, the present invention relates to a defect inspection apparatus for detecting phase shifter defects on a phase shift mask.
With the achievement of miniaturization of semiconductor integrated circuits, circuit elements and wiring patterns have become small and fine to such an extent that the design rule therefor are of the order of submicrons. Under these circumstances, a reduction in the pattern transfer accuracy is a serious problem in a photolithography process wherein an integrated circuit pattern on a photomask is transferred onto a semiconductor wafer by light of i-line (wavelength 365 nm), KrF-line (wavelength 245 nm), etc. Accordingly, a photolithography process requiring a particularly high resolution uses a phase shift (photo) mask in which phase shifters are provided at light-transmitting portions.
The principal structure of phase shift masks is a substrate engraving type phase shift mask produced by engraving a glass substrate to form phase shifters over light-transmitting portions of the mask. A sectional view of such a substrate engraving type phase shift mask is shown in FIG. 2. On a glass transparent substrate 31, a light-shielding pattern 32 of chromium is provided in the form of a repeated pattern, and the transparent substrate 31 is etched at alternate space regions between the adjacent portions of the light-shielding pattern 32 to a depth corresponding to a half of the working wavelength (about 180° in terms of phase difference) to form trenches 33.
Phase shifter defects on such phase shift photomasks may be generated in the mask making process. Examples of phase shifter defects are a partially unfinished phase shifter due to adhesion of a contamination to a region where a 180° phase shifter is to be formed, and an excess or overetched phase shifter due to a resist pinhole or the like present in a region where no phase shifter should be formed.
The edges of phase shifter forming regions are formed under the chromium pattern, and the phase shifter is formed at a light-transmitting portion. For these reasons, all the above-described phase shifter defects cannot be detected by a conventional inspection method using transmitted light or a conventional inspection method in which both the front and back surfaces of a phase shift photomask are illuminated with light and the reflected light and the transmitted light are compared with each other. Therefore, the following inspection method is presently employed. The pattern of a photomask under inspection is transferred onto a wafer by an exposure system, and whether or not there is a defect is checked by an inspection machine using the transferred pattern.
However, in the case of a semiconductor integrated circuit device having a line width of 0.1 μm or less, the pattern defect detection size on the wafer is 10 to 30 nm. Therefore, pattern defects cannot be detected with a wafer defect inspection apparatus.
Under the above-described circumstances, it has become necessary to develop an inspection apparatus for detecting phase shifter defects on a substrate engraving type phase shift mask.
The conventional techniques and problems associated therewith are as follows:    1) Because phase shifters are formed at light-transmitting portions of a mask, it is impossible to detect a phase difference-deviating step defect formed on the glass at a light-transmitting portion by using transmitted light inspecting type inspection apparatus commercially available from KLA-Tencor, Lasertech, etc.    2) Regarding the defect inspection of a substrate engraving type phase shift mask in particular, because the edges of phase shifter forming regions is formed under the chromium pattern, phase shifter defects cannot be detected even by an inspection method in which comparison is made between a transmitted light image from the back surface of the mask and a reflected light image from the front surface of the mask at an arbitrary region in an inspected portion of the mask, e.g. the STARlight (trade name: KLA-Tencor) inspection method, which is another inspection method usable by mask inspection apparatus.    3) With inspection apparatus using reflected light from the front surface where the mask pattern is formed, e.g. 9MD83SR, available from Lasertech, it is impossible to detect defects in a case where the amount of phase difference is small, i.e. 120° or less.    4) As has been stated above, the mask inspection apparatus commercially available at present cannot satisfactorily detect phase shifter defects. For this reason, the conventional practice is to employ the following method to check whether or not there is a defect in a phase shift mask. That is, the pattern of a photomask under inspection is transferred onto a wafer by a wafer exposure system, and the presence of a defect in the phase shift mask is judged by a 2-chip comparing inspection apparatus using the transferred pattern. Accordingly, evaluation cannot be conducted in the mask making process. The wafer process is requested to perform evaluation, resulting in an increase in the number of man-hours needed to carry out the mask making process.    5) Moreover, when circuit patterns become finer, i.e. 0.1 μm or less in line width, in the future with the achievement of further miniaturization, the size of a defect is 10 to 30 nm when such a fine pattern is transferred onto a wafer to evaluate the phase shift mask. The detection sensitivity of the present wafer defect inspection apparatus is not sufficient to inspect the phase shift mask for such small defects. Accordingly, there will be no means for evaluating phase shift masks.